|Mica Wickersham (mica)
Post Number: 250
|Posted on Tuesday, September 10, 2002 - 5:56 pm: |
The pickups on the Series I and II basses and guitars are single coil. They employ an electronic humcancelling circuit to reduce the hum inherent with single coil pickups.
You will evaluate the hum of each pickup independently, so this test procedure should first be done with the pickup selector switch on one pickup only, then test the other pickup only. The procedure won’t work if you listen to the pickups at the same time.
The humcancelling trimpots are the center two trimpots accessible through the backplate of the instrument. First, determine which humcancelling trimpot controls the pickup you’ve started with. With one pickup selected, move one humcancelling control back and forth. If you hear no effect, then use the other humcancelling trimmer for the first calibration.
The how to:
1] Turn the humcancelling trimpot to the most counter clockwise rotation.
2] Rotate the entire guitar in space to the position where the hum is the loudest. Note: don't put the instrument right up against an amplifier, keep it a respectable distance from everything like you would for playing a song.
3] Now adjust the trimpot through the entire clockwise rotation to hear how the hum passes through a null and then gets louder again. You’ll probably find a position that is near the center or slightly clockwise of center that nulls the hum for most situations.
4] Repeat this procedure for the other pickup.
The original Series I/II (PF6) circuit is effective over the frequency range up to a few hundred Hz, and is very effective for your mains frequency (usually 50 or 60Hz) and its first few harmonics. This was the typical
interference environment when the system was designed. But over the years, higher frequency harmonics from solid-state lamp dimmers and switching power supplies have added an annoying hi frequency buzz which consists of much higher harmonics of the mains frequency.
This high frequency noise which survives even the most careful adjustment of the humcancelling trimpot can be reduced by more than 20dB but requires the electronics get upgraded to current specs. Persistent high frequency noise or a directional noise requires the electronics get upgraded to current specs. Contact Alembic at 707.523.2611 if your instrument requires this servicing.